In commercial paint spray systems, it is typical to provide a spray booth and an exhaust system for withdrawing excess paint vapors from within the confines of the spray booth. The exhausted air and vapors are initially cleaned by any of several known cleaning means and is then dumped outside of the spray booth and typically ever to the atmosphere outside the building or enclosure housing the spray booth. A fan or blower is used to draw enclosure air through the confines of the spray booth and to force this air and the excess paint vapors through the cleaning means and up the exhaust duct to discharge. Known exhaust systems typically have a damper control for shutting off the exhaust duct responsive to a fire sensing means detecting a fire. It is important to close the damper control in the event of a fire, since the smoke otherwise rises through the open duct to feed the fire, and further the fire itself can rapidly spread through the duct system to the roof of the building.
The exhaust system need really be operated only when there is contaminating excess paint vapors actually in the confines of the spray both, and operation at other times can add needlessly to the operating costs of the exhaust system. This would by both from the standpoint of the electric power needed to operate the exhaust fan for exhausting the air and vapors through the system, and also from the standpoint of the wasted energy that is used to heat the enclosure air that is then just dumped to the atmosphere. Even with the exhaust system deenergized, the convective air flow up the open exhaust duct can allow the heated enclosure air to wastefully escape to atmosphere.